Ablation therapy is a medical procedure where dysfunctional tissue is ablated using various forms of energy, typically in the form of extreme heat or cold. Ablation therapy is utilized to treat tumors in lung, liver, kidney, bone and in other body organs as well as in the treatment of cardiac rhythm conditions such as Atrial Fibrillation. Procedures are typically performed under image guidance, such as X-ray screening, CT scan or ultrasound by an interventional radiologist or cardiac electrophysiologist.
Although ablation treatments are useful, it is difficult to determine with sufficient accuracy the parameters needed for successful treatment. Inexactness in the amount of energy or exposure time of an affected tissue may lead to thermal injury of the adjacent healthy tissues. Catheter ablation of the heart is particularly susceptible to such problems.
Radio-frequency (RF) catheter ablation, for example, is commonly used to treat atrial fibrillation (AF) which is the most common heart arrhythmia leading to hospitalization. A catheter is inserted into a patient's heart or other vessel, and heat is applied to a localized region until the tissue in that region has been sufficiently destroyed to abate the arrhythmia. In other applications, cryoablation has also been used to freeze and destroy local tissue.
The use of extreme energy during cardiac catheter ablation procedures for the treatment of atrial fibrillation is prone to a serious and life-threatening complication known as atrioesophageal fistulas. Atrioesophageal fistula after catheter ablation occurs due to conductive heat transfer to the esophagus that causes transmural tissue necrosis. The close proximity of the esophagus to the posterior wall of the left atrium and the pulmonary veins presents a significant risk of injury to the esophagus during the application of energy to the cardiac tissue. Injury to the esophagus resulting in tissue necrosis can create a delayed opening in the esophageal wall, leading to the formation of a fistula between the atrium and the esophagus. Atrioesophageal fistulas, if not diagnosed and treated promptly, may lead to, infection and sepsis, bleeding, air and particulate-matter emboli, stroke and quite often death.
To date there have been no effective measures to prevent atrioesophageal fistula formation. Various techniques are employed to minimize the likelihood of esophageal injury during percutaneous catheter ablation. Many physicians avoid ablating the posterior wall of the left atrium and pulmonary veins adjacent to the esophagus to reduce the likelihood of injury to the esophagus. Techniques such as altering the lesion set by moving ablation lines away from the areas adjacent to the esophagus add to the difficulty of treating the fibrillation. Physically moving the esophagus away from the heart wall with a luminal transesophageal echo probe is also employed. These techniques are dependent on the specific anatomical location of the esophagus relative to the area being ablated. With no thermal feedback from the esophagus, the physician has no guarantee that energy is not spreading to the esophageal tissue. Atrial fibrillation recurrence rates are thought to be significantly higher when these types of avoidance techniques are employed.
Titration of energy is the most common method employed to minimize risk of esophageal injury during percutaneous catheter ablation. The challenge of this approach is in knowing how much energy can be delivered before injury occurs to the esophagus. Typically the energy that is transferred to the esophagus is measured with a luminal esophageal temperature monitoring catheter. These catheters are placed down the esophagus of the patient and provide a single-point measurement of the temperature at the tip of the catheter. The premise is that this thermal feedback will provide the Electrophysiologist with sufficient information to allow for the proper titration of energy and eliminate risk of injury to the esophagus.
Several challenges limit the effectiveness of luminal esophageal temperature monitoring devices during catheter ablation. Studies employing luminal esophageal temperature monitoring devices reveal that esophageal heating occurs in the range of 0.05-0.1 degrees Celsius (C) per second and that repeated energy applications in the same general area can cause temperature stacking. The physician must position the temperature monitoring device adjacent to the ablation catheter before each pulse of energy. This is very time consuming and difficult to achieve under x-ray guidance. The temperature monitoring catheters are very small in diameter relative to the diameter of the esophagus. It is nearly impossible to position the tip of the temperature probe against the esophageal wall that is adjacent to the area of the heart wall being ablated. Furthermore, the temperature-monitoring catheters are not designed to be torqued or deflected toward the esophageal wall and cannot be positioned precisely within the lumen. A recent study showed that over 6% of patients exhibited evidence of esophageal ulceration after catheter ablation when currently available luminal temperature monitoring products were used and many cases of atrioesophageal fistulas have been documented despite the use of luminal esophageal temperature monitoring devices.
As catheter ablation for the treatment of Atrial Fibrillation expands beyond the premier academic institutions and into the mainstream, the limitations of today's available options for protecting against esophageal injury will become more evident. More physicians will be forced to make the trade-off between sufficient ablation and the potential for damage to the patient's esophagus. In addition to the complications related to esophageal injury, the lack of adequate feedback will result in longer procedure times, excess radiation exposure, and increased arrhythmia recurrence rates.
There is a clear need for improved devices, systems and methods to monitor temperature while actively ablating target tissue in order to achieve the desired clinical outcome without risk of injury to the surrounding healthy tissues.